This proposal is based on previous observations that low doses of intermittent parathyroid hormone (PTH) increase bone mass in vivo. Knowledge of mechanisms and mediators which regulate the anabolic effect of PTH will permit a more rational approach to using PTH to treat osteoporosis, and may suggest new approaches to increasing bone mass to compensate for pathologic bone loss. We hypothesize that the anabolic effect of PTH in vivo is mediated by growth hormone-dependent insulin-like growth factor I (IGF-I). The goal of the first study is to determine if PTH selectively increases mRNA and protein for IGF-I and IGF-binding proteins (IGF-BPs) in trabecular bone of young intact and hypophysectomized (HX) rats treated with vehicle or PTH alone or in combination with GH for 1, 3, 5 or 12 days. distal femurs will be separated into trabecular, cortical and marrow compartments and changes in IGF-I will be correlated in time with possible changes in mRNA for PTH receptor, alternate putative mediators, IGF-II, TGFbeta and PGH synthase, matrix proteins, procollagen 1alpha(I) and osteocalcin, measured using semi-quantitative RT-PCR, solution- hybridization/nuclease protection assays or Northern blot hybridization. Changes in these mRNAs with time will be related to the increase in percent of bone forming surfaces. In the second study, in situ histohybridization will be used to detect the spatial expression of the selected mRNAs and to determine the relative locations of cells expressing growth factor mRNA and those expressing the PTH receptor or the bone matrix proteins in trabecular bone of lumbar vertebrae. Spatial location of trabecular bone cells secreting IGF-I and IGF-BPs will be identified using immunocytochemistry. the goal of the third study is to determine if IGF-I monoclonal antibody or excess IGF-BPs will block the anabolic effect of PTH when infused locally into one distal femur in vivo. Bone density will be monitored by quantitative radiography; bone mineral content will be measured by single photon absorptiometry (SPA); and bone formation by bone histomorphometry. The goal of the fourth study is to determine if there are shifts in time or location in mRNA for IGF-I, IGF-II, IGF-BPs, TGFbeta, PGH synthase and PTH receptor or in protein synthesis of IGF-I, IGF-BPs or PGH synthase in animal models, such as thyroid-parathyroidectomized rats infused with PTH or aged rats given intermittent PTH, in which we have shown the anabolic response to PTH to be absent or blunted.